I heard some colleagues brainstorming about this. One option is to add something in your layout that checks if there is an object in, say, entity 'Notification' that is readable for all users and acts on it. But that requires a database request on each page load. Another option is to build a widget/module that makes use of sockets. It doesn't exist yet. I'd be interested too in an easy way to let users know they'll be kicked shortly.
May be vote for this idear? https://forum.mendix.com/link/ideas/533